The present invention relates to a control system for a hydraulic pressure regulating valve in a hydraulic control circuit of an automatic transmission connected to a multi-cylinder internal combustion engine which is provided with means whereby the engine may run on selected cylinders of all under light load operating condition.
It is known for the purpose of improving fuel economy to operate a multi-cylinder internal combustion engine on selected cylinders of all under light load condition. It is also known to cut off fuel supply to some cylinders of all, under light load engine operating condition, to let the engine run on the remaining selected cylinders.
In regard to an automatic transmission connected to a multi-cylinder internal combustion engine, it is known that, for the control of the automatic transmission, a line pressure, i.e., a constant hydraulic pressure, is reduced by draining hydraulic fluid through a hydraulic pressure regulating valve (a throttle valve) in response to induction vacuum within the engine intake manifold to provide a throttle pressure, i.e., a hydraulic pressure representing load imposed on the engine.
Explaining into the detail in connection with FIG. 1, induction vacuum within the engine intake manifold is always applied to a diaphragm device 1 to a chamber 2 thereof so that, against a spring 3 biasing a push rod 4 in a direction to push a valve member or spool 4 of a throttle valve 5, the diaphragm device 1 will pull the valve spool 4 toward the right, viewing in this Figure, in response to a pressure difference between the atmospheric pressure and the intake vacuum of the engine. As the intake vacuum increases, a pushing force by the rod 4 pushing the valve spool 6 toward the illustrated position will decrease, thus reducing magnitude of the throttle pressure. Thus, the throttle pressure will represent the load on the engine. However, with the conventional control system in which the diaphragm chamber 2 communicates only with the engine intake manifold, the magnitude of the throttle pressure will no longer represent the actual load on the engine when the engine runs on selected cylinders of all. This problem will be described hereinafter.
In the case of a 6-cylinder internal combustion engine, under the same running condition, the magnitude of induction vacuum is very small when the engine runs on three cylinders of all as compared to that when it runs on all six cylinders thereof (see graph shown in FIG. 2). As a result, the pushing force by the rod 4 will become excessively high when the engine runs on the three cylinders of all, thus increasing the magnitude of the throttle pressure. This will result in that the shifting will not take place at appropriate timings when the engine runs on the selected cylinders of all.